I just have the Mirt the Moneylender's First Bank of Waterdeep's Bottomless change purse. (2.500 gp)
It holds an unlimited amount of coins and cut gems, but only coins and cut gems. If you pay for the upgrade (10,000 gp), your pouch is attached to Mirt's demi-plane of banking where your money can only be withdrawn by the account owner when he isn't being mind-controled. The demi-plane of banking performs automatic exchanges and provides cash in whatever denomination you desire. Mirt is very progressive in that he offers 0% interest on all accounts, with the purchase of a Greater Bottomless Change Purse. (ie, he doesn't charge you to keep your money safe, unlike most other Moneylenders.) And since it's backed by Waukeen, goddess of merchants, you don't want to even know what sort of horrible curses will befall anyone who tries to steal from the Demi-plane of Banking.